1. Field of the Invention
The present invention relates to a quartz crystal oscillator in which a quartz crystal resonator is inserted into a feedback loop of an amplifier for oscillation to reduce phase noise, and more particularly to a crystal oscillator having excellent starting characteristics.
2. Description of the Related Arts
Crystal oscillators are often used in high-performance radio equipment because they have excellent oscillation frequency stability or the like.
The assignee of the present invention has already proposed a crystal oscillator that reduces phase noise in an oscillation output thereof in Japanese Patent Laid-Open Publication No. H09-153740 (JP, 9-153740A). FIG. 1 shows an example of the circuit configuration of a crystal oscillator of the prior art.
A crystal oscillator is basically composed of resonance circuit 1 and amplifier 2 for oscillation. Resonance circuit 1 is composed of quartz crystal unit 3 as an inductor component, and split capacitor 4a, 4b. Capacitors 4a, 4b are connected together in a series, and crystal unit 3 is parallel-connected to the series-connected pair of capacitors 4a, 4b. Amplifier 2 includes a transistor for oscillation, and this transistor has its base connected to the node of capacitor 4a and crystal unit 3, its emitter connected to the mutual node between capacitors 4a, 4b, and its collector connected to power supply Vcc. Amplifier 2 for oscillation feeds back and amplifies an oscillation frequency component that depends on resonance circuit 1. The emitter is both connected to output terminal Vout and to one end of load resistor R3. The other end of load resistor R3 is connected to the node of capacitor 4b and crystal unit 3. Output line 5 that makes up a portion of the feedback loop is further provided such that the node of capacitors 4a and 4b is connected to the emitter. Crystal resonator 6 is inserted into output line 5. Finally, bias resistors R1, R2 are provided for applying a bias voltage to the base of the transistor.
In this crystal oscillator, the resonance frequency of crystal resonator 6 is caused to generally match the oscillation frequency of the crystal oscillator. In this way, only the fundamental wave component of oscillation frequency f passes through crystal resonator 6, whereby the output signal becomes a narrow band and phase noise in the output signal can be reduced to a low level. In other words, crystal resonator 6 is used as a filter for eliminating spurious components in the oscillation output and extracting only the fundamental wave component.
To bring about a further stabilization of the oscillation frequency in this type of crystal oscillator, crystal unit 3 is normally accommodated in a thermostatic oven and thus caused to operate in a fixed-temperature state.
Curved line A of FIG. 2 shows the relation between temperature and frequency deviation Δf/f where f is the nominal oscillation frequency of the crystal unit, and Δf is the deviation of the actual oscillation frequency from the nominal oscillation frequency f. Typically, the frequency deviation exhibits a change that is represented by a cubic function curve with respect to temperature. As can be seen in this graph, the crystal unit has a minimum value of the oscillation frequency in the vicinity of 70° C., and the internal temperature of the thermostatic oven that accommodates crystal unit 3 is therefore normally set to the minimum value of approximately 70° C.
However, because the crystal oscillator of the above-described configuration uses a thermostatic oven, some time interval is required following the introduction of the power supply for the temperature in the thermostatic oven to reach a temperature of, for example, 70° C. During this time interval, the oscillation frequency will vary and the oscillation itself will be unstable, and this crystal oscillator therefore suffers from the problem of having a poor starting characteristic.
The crystal resonator also has a frequency-temperature characteristic that is a cubic function curve, as with the crystal unit. The crystal resonator is arranged outside the thermostatic oven, and the crystal resonator is designed such that when the temperature of the crystal resonator is at room temperature and the temperature of the crystal unit is, for example, 70° C., the resonance frequency of the crystal resonator will coincide with the fundamental wave component of the oscillation frequency of the crystal unit. However, the problem arises that if the temperature of the crystal unit is in the vicinity of room temperature immediately following the introduction of the power supply, as a result, the oscillation frequency due to the crystal unit will differ greatly from the resonance frequency of the crystal resonator, the fundamental wave component of the oscillation frequency will not pass through the crystal resonator, and the circuit will not oscillate.
The crystal resonator can also conceivably be accommodated in a thermostatic oven, but such a measure would necessitate a larger thermostatic oven in the crystal oscillator, and would both increase power consumption and hinder miniaturization of the crystal oscillator. In addition, the frequency-temperature characteristic of the crystal unit does not necessarily match the frequency-temperature characteristic of the crystal resonator, and in such a case, the resonance frequency of the crystal resonator and the oscillation frequency due to the crystal unit may differ depending on the ambient temperature even when the temperature of the two components is the same, and the possibility therefore exists that the circuit will not oscillate for the same reasons as described hereinabove.